In the petroleum industry, it is common for gasoline fuels to become contaminated with sulfur. Engines and vehicles utilizing sulfur-contaminated fuels can produce harmful emissions of nitrogen oxide, sulfur oxide and particulate matter. Government regulations have become more stringent in recent years with regard to allowable levels of these potentially harmful emissions, which has led refiners to seek ways to reduce sulfur levels in these fuels.
Gasoline fuel is generally prepared by blending several petroleum fractions. Typical refineries blend, among other blendstocks, catalytically cracked gasoline (CCG), coker gasoline, straight run naphtha, reformats, isomerate and alkylate to produce gasoline fuel having pre-designed specifications. Among such various blendstocks, CCG (which is produced from fluidized catalytic cracking) is responsible for a substantial portion of the sulfur content in the resulting blended gasoline pool. Therefore, removal of sulfur compounds contained in CCG is an important step in meeting the rigorous regulations on sulfur content in gasoline fuel.
Various methods have been proposed to reduce sulfur levels in these CCG-containing fuels. However, there are disadvantages associated with these previously proposed methods. In general, removal of sulfur compounds from CCG-containing petroleum fractions is accomplished by catalytic hydrodesulphurization, whereby the petroleum fractions are contacted with solid catalyst in the presence of hydrogen gas. Hydrogen disulfide is a product of certain of these reactions. Typical hydrodesulphurization catalyst consists of alumina support, molybdenum sulfide, cobalt sulfide and/or nickel sulfide. The cobalt sulfide and/or nickel sulfide are added to the catalyst in order to increase catalytic activity and selectivity.
There are disadvantages or limitations to using hydrodesulphurization alone for sulfur removal. For example, sulfur compounds contained in petroleum streams have a wide variety of reactivity in catalytic hydrodesulphurization. Bruce C. Gates (Ind. Eng. Chem. Res. Vol. 30, pp. 2021-2058, 1991) indicated that pseudo first order reaction rates of hydrodesulphurization for thiophene, benzothiophene, and dibenzothiophene are known to be 100, 59, and 4, respectively, although extent of such differences depends on the chemical composition, for example, olefin content, in feedstock. Additionally, alkyl group substituents on thiophenic and benzothiophenic molecules diminish the reactivity of those molecules in hydrodesulphurization. Therefore, much higher temperatures and hydrogen pressures are required to hydrodesulphurize CCG-containing petroleum feedstocks containing alkylated thiophenic, benzothiophene, and alkylated benzothiophenic compounds than feedstock containing thiophenic compounds only.
Along with high temperature and high pressure hydrodesulphurization, hydrogenation of other compounds in the CCG feedstock, including the carbon-carbon bonds of olefinic compounds, also occurs. Olefinic compounds contained in CCG contribute significantly to the high octane rating of the feedstock. Hydrogenation of these olefinic compounds to paraffinic compounds results in a lowering of octane rating which is undesirable for automobile applications of gasoline. Significant loss of octane rating during catalytic hydrodesulphurization of CCG must be compensated through blending substantial amounts of reformate, isomerate and alkylate into the gasoline pool, which is detrimental to the economy of the refining process.
Olefinic compounds are concentrated in low boiling point range fractions of CCG, while sulfur compounds are concentrated in high boiling point range fractions of CCG. Therefore, certain prior art patents show separate processing of low boiling point and high boiling point fractions of CCG.
For example, U.S. Pat. No. 6,623,627 involves fractionating feed gasoline into three streams, each of which is subsequently treated by a different method to attain low sulfur gasoline without severe hydrogenation of olefinic compounds. U.S. Pat. No. 6,303,020 involves catalytic distillation and inter-stage H2S removal to maintain high octane rating and low sulfur content in the product gasoline. U.S. Pat. No. 6,334,948 involves separating feed gasoline into light and heavy fractions and then treating each fraction with different catalysts. U.S. Pat. No. 6,610,197 involves separating catalytically cracked naphtha into light and heavy fractions and then treating the fractions to obtain low sulfur gasoline product. In particular, U.S. Pat. Nos. 6,334,948 and 6,610,197 utilize fractionation as an initial step followed by catalytic hydrogenative desulfurization.
None of these methods, however, achieve the desired sulfur reduction and substantially similar octane levels economically, i.e., at low temperature and low hydrogen pressure levels and milder reaction conditions, or prevent a significant amount of hydrogenation of olefinic compounds when used alone. Furthermore, CCG having a high end boiling point is very difficult to desulphurize due to its high sulfur content. Therefore, undercutting of CCG to have low sulfur content has been recognized as a means for deep desulphurization, although it decreases the production amount of valuable gasoline.
Catalysts which have high selectivity toward hydrodesulphurization rather than hydrogenation of olefinic compounds have been also proposed. An example of such a prior art catalyst is molybdenum sulfide supported on neutral alumina. However, these catalysts are designed to have higher selectivity toward hydrodesulphurization of sulfur compounds rather than hydrogenation of olefinic compounds and thus, sacrifice hydrodesulphurization activity to suppress hydrogenation activity, which is not suitable for practical application.
Non-catalytic methods to remove sulfur compounds from gasoline feedstock have also been proposed to prevent the loss of octane rating that typically accompanies catalytic hydrodesulphurization. Examples of representative non-catalytic desulphurization methods typically include using adsorbents such as zeolite to selectively remove certain specific sulfur compounds contained in gasoline feedstock. However, zeolitic adsorbent is very difficult to regenerate. Also, certain of these prior art methods are directed only towards treating those portions of gasoline having concentrated sulfur compounds, or only towards certain types of fuels such as diesel fuels. Additionally, the industry recognizes that there is very difficult to remove large amounts of sulfur compounds contained in feed CCG to be less than a few tens of weight ppm level.
Further, non-catalytic removal of sulfur compounds requires large amounts of reagent and its storage and recycle devices, which can be economically unfeasible, and is often capable of removing only certain specific types of sulfur compounds when used alone, which makes its application limited for use in a broad range of industrial processes. Further, certain adsorption technologies, in particular gas phase adsorption, consume prohibitively high amounts of energy.
It would be beneficial to have a process for obtaining gasoline having reduced sulfur content by mild hydrodesulphurization without the need for post treatment even when using CCG having a high end boiling point and/or high sulfur content. It would also be beneficial to have a process for simple adsorptive treatment of CCG feedstock to achieve deep hydrodesulphurization of CCG without severe hydrogenation of olefinic compounds, in order to maintain a high octane rating of CCG feedstock. It would also be beneficial to have a process which allows partial removal of specific sulfur compounds from a CCG gasoline feedstock having a full boiling point range via adsorption such that the adsorbent can have a long run length until saturation.